1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus, and in particular, to a technique for correcting a positional displacement of a sheet in a width direction thereof.
2. Description of the Related Art
A typical image forming apparatus, such as a copier, printer, and a facsimile machine, has a sheet conveying device that conveys a sheet to an image forming portion for forming an image on the sheet.
The sheet conveying device has a skew correcting portion to correct a skew of a sheet and conveys the sheet to the image forming portion. One example technique used in the skew correcting portion is to correct a skew by detecting the amount of skew of a sheet and changing the ratio between revolutions of a pair of conveying rollers arranged in a sheet width direction, to which a sheet conveying direction is perpendicular, or the number of revolutions of each of the conveying rollers.
A positional displacement may occur in a sheet in the sheet width direction during conveyance by the time the sheet reaches the image forming portion. To address this, some sheet conveying devices have a positional displacement correcting portion that corrects the positional displacement by detecting the positional displacement in the sheet width direction using, for example, a sensor, calculating the amount of displacement from a conveyance reference, and shifting the pair of conveying rollers conveying the sheet in the width direction while pinching the sheet according to the calculated amount of displacement.
A sheet conveying device including a skew correcting portion that has a pair of conveying rollers, as described above, first corrects a skew of a sheet using the skew correcting portion. Then, the sheet conveying device corrects a displacement in the sheet width direction using the positional displacement correcting portion. In this way, the sheet is conveyed to an image forming portion along a conveyance reference. One example of such a technique is disclosed in Japanese Patent Laid-Open No. 2002-019999.
A positional displacement correcting portion for correcting a displacement in the sheet width direction in such a known sheet conveying device is composed of a pair of conveying rollers capable of being axially shifted downstream of a skew correcting portion in the sheet conveying direction. The pair of conveying rollers are made to stay at a conveyance reference position, pinch a sheet at that position, and then are shifted in the axial direction based on the amount of displacement from a conveyance reference detected by, for example, a sensor. In this way, the sheet is moved to the conveyance reference position, and a displacement in the sheet width direction is corrected.
FIGS. 17A to 17D illustrate a positional displacement correcting portion in such a known sheet conveying device. In FIGS. 17A to 17D, conveying rollers 305 can be shifted in the axial direction (width direction). A sheet Sa has a length in the width direction of, for example, approximately 330 mm, which corresponds to a size larger than a normal A4-size or A3-size sheet. A sheet Sb is the smallest sheet conveyable by the known sheet conveying device, for example, a B5R-size sheet, whose length in the width direction is approximately 182 mm.
It is necessary to arrange the conveying rollers 305 such that they can be axially shifted while pinching a sheet of all conveyable sizes, so they are arranged so as to conform to the small-size sheet Sb. For example, as illustrated in FIGS. 17A and 17B, the conveying rollers 305 are arranged at a distance x away from a conveyance reference (indicated by dashed lines). The conveyance reference is a line that passes through the center of a conveying path in the width direction. A sheet is conveyed while the center of the sheet in the width direction is aligned with the conveyance reference.
In such a positional displacement correcting portion, when the large-size sheet Sa is conveyed while being largely displaced in the width direction, as illustrated in FIG. 17C, the displacement of the sheet Sa is corrected by movement of the conveying rollers 305 toward the front side in the width direction. The amount of movement of the conveying rollers 305 at this time corresponds to the amount for matching the center of the sheet in the width direction with the conveyance reference. To this end, the positional displacement of the sheet in the width direction is detected by a sensor (not shown), the amount of displacement from the conveyance reference is calculated, and the conveying rollers 305 are moved based on the calculated amount of displacement.
At this time, the conveying rollers 305 are shifted while pinching the large-size sheet Sa at an offset position in the width direction. In this case, the frictional force between the guiding surface of the conveying path and the large-size sheet Sa unbalances the shifting force. That is, because the forces applied to the left and right conveying rollers 305 from the sheet caused by the frictional force are different from each other, slippage occurs between the conveying rollers 305 and the sheet Sa. Thus, the sheet Sa cannot be shifted by the accurate amount. In addition, the frictional force to the guiding surface of the conveying path may cause the sheet to be oblique.
In the case of the small-size sheet Sb, when a displacement of the sheet in the width direction is large, a side end of the small-size sheet Sb may be positioned inside the outer edge of one of the conveying rollers 305, as illustrated in FIG. 17D. In this case, because both the frictional forces to the guiding surface of the conveying path and the forces of pinching the sheet are different between the left and right conveying rollers 305, the lateral shifting force is unbalanced. As a result, slippage occurs and the sheet Sb cannot be shifted by the accurate amount.
As described above, when the conveying rollers 305 are arranged leftward and rightward at a distance x from a sheet conveyance reference, there is a possibility that a displacement is not corrected with high precision.